The present disclosure relates to technology for non-volatile storage.
One example of non-volatile memory uses resistance-switching memory elements. A variety of materials exhibit reversible resistance-switching behavior. These materials include chalcogenides, carbon polymers, perovskites, and certain metal oxides and nitrides. Specifically, there are metal oxides and nitrides which include only one metal and exhibit reliable resistance switching behavior. This group includes, for example, NiO, Nb2O5, TiO2, HfO2, Al2O3, MgOx, CrO2, VO, BN, and AlN.
A layer of one of these materials may be formed in an initial state, for example a relatively low-resistance state. Upon application of sufficient current or voltage, the material switches to a stable high-resistance state. This resistance switching is reversible such that subsequent application of an appropriate current or voltage can serve to return the resistance-switching material to a stable low-resistance state. This conversion can be repeated many times. For some materials, the initial state is high-resistance rather than low-resistance.
A rewriteable non-volatile memory cell may be formed that includes a diode coupled in series with a reversible resistance-switching material such as a metal oxide or metal nitride. However, operating memory devices that employ reversible resistance-switching materials is difficult. Furthermore, operating memory devices that employ reversible resistance-switching materials to store multiple bits of data is difficult.
Some devices employ reversible resistance-switching material that is used to store two ranges of resistance and, therefore, the memory cell can be programmed/erased between two states, an erased state and a programmed state that correspond to data “1” and data “0.” Such a device is referred to as a binary or two-state device.
A multi-level memory cell is implemented by identifying multiple, distinct allowed ranges of resistance (or other property). Each distinct range corresponds to a predetermined value for the set of data bits. The specific relationship between the data programmed into the memory cell and range of resistance depends upon the data encoding scheme adopted for the memory cells.